Field of the Invention
This invention relates generally to a heat-assisted magnetic recording (HAMR) type of magnetic recording hard disk drive, and more particularly to a HAMR disk drive having a magnetic recording disk with multiple continuous magnetic recording layers.
Description of the Related Art
In conventional magnetic recording hard disk drives, the recording layer on the disk is a continuous film of magnetic material. The recorded data “bits” are composed of multiple weakly-coupled neighboring magnetic grains that form a single magnetic domain and are physically adjacent to one another. In contrast to continuous media, patterned media, also called “bit-patterned media” or BPM, have been proposed wherein the magnetic material in the recording layer is patterned into small isolated data islands. Each island contains a single magnetic “bit” and is separated from neighboring islands by a nonmagnetic region.
In conventional continuous magnetic recording, thermal instabilities of the stored magnetization in the recording media can cause loss of recorded data. To avoid this, media with high magneto-crystalline anisotropy (Ku) are required. However, increasing Ku also increases the coercivity of the media, which can exceed the write field capability of the write head. Since it is known that the coercivity of the magnetic material of the recording layer is temperature dependent, one proposed solution to the thermal stability problem is heat-assisted magnetic recording (HAMR), wherein high-Ku magnetic recording material is heated locally during writing to lower the coercivity enough for writing to occur, but where the coercivity/anisotropy is high enough for thermal stability of the recorded bits at the ambient temperature of the disk drive (i.e., the normal operating or “room” temperature of approximately 15-30° C.). In some proposed HAMR systems, the magnetic recording material is heated to near or above its Curie temperature, i.e., the temperature above which the material becomes paramagnetic. The recorded data is then read back at ambient temperature by a conventional magnetoresistive read head. HAMR disk drives have been proposed for both conventional continuous media and for BPM.
In a typical HAMR write head, light from a laser diode is coupled to a waveguide that guides the light to a near-field transducer (NFT) (also known as a plasmonic antenna). A “near-field” transducer refers to “near-field optics”, wherein the passage of light is through an element with sub-wavelength features and the light is coupled to a second element, such as a substrate like a magnetic recording medium, located a sub-wavelength distance from the first element. The NFT is typically located at the air-bearing surface (ABS) of the air-bearing slider that also supports the read head and magnetic write pole and rides or “flies” above the disk surface. NFTs are typically formed of a low-loss metal (e.g., Au, Ag, Al, Cu) shaped in such a way to concentrate surface charge motion at a notch or tip located at the slider ABS when light is incident. Oscillating tip charge creates an intense near-field pattern that heats the recording layer on the disk. The magnetic write pole is then used to change the magnetization of the recording layer while it cools. Sometimes the metal structure of the NFT can create resonant charge motion (surface plasmons) to further increase intensity and disk heating. For example, when polarized light is aligned with an E-antenna type of NFT, an intense near field pattern is created at the notch or tip of the E-antenna. Resonant charge motion can occur by adjusting the E-antenna dimensions to match a surface plasmon frequency to the incident light frequency. A NFT with a generally triangular output end, sometimes called a “nanobeak” type of NFT, is described in U.S. Pat. No. 8,705,325 B2 and U.S. Pat. No. 8,705,327 B2, both assigned to the same assignee as this application. In this type of NFT an evanescent wave generated at a surface of the waveguide couples to surface plasmons excited on the surface of the NFT and a strong optical near-field is generated at the apex of the triangular output end.
Multilevel HAMR media and disk drives have been proposed to increase the data density, but for BPM. In multilevel HAMR BPM each data island is formed of multiple stacked cells, so the data density is increased by a factor of 2(n−1), where n is the number of cells in each island. For example, if there are two cells in each island the recorded bits in the two cells are thus aligned vertically to provide a composite readback signal which is decoded into one of 4 possible values. U.S. Pat. No. 8,081,542 B1, assigned to the same assignee as this application, describes a disk drive with multilevel HAMR BPM having two stacked magnetic cells in each data island.
What is needed is a HAMR disk drive with disks having multiple continuous non-patterned magnetic recording layers each of which stores an independent data stream, and with a read channel that can detect the independent data steams from a composite readback signal.